FIG. 24 is a cross-sectional view showing the main components of a conventional magnetic recording/playback device, and FIG. 25 is a sectional view along line S25-S25 in FIG. 24.
In the figures, a reference numeral 1 designates a fixed lower drum; 2 designates a bearing mounted to this lower drum; 3 designates a shaft that rotates supported by bearing 2; 4 designates a base block firmly attached to the rotary shaft 3; 5 designates a rotating upper drum mounted to base block 4 by means of a screw 6; 7 designates an actuator mounted to upper drum 5 by means of a screw 8; 9 designates a lower transformer mounted to lower drum 1; 10 designates an upper transformer mounted ot base blcok 4; 11 designates a wiring board mounted to upper drum 5; 12 designates a non-rotary contact for the purpose of supplying a control current to actuator 7; 13 designates a rotary electrode disposed on a portion of base block 4 to provide slidable contact with contact 12; and 14 designates a connector that provides electrical conductivity from electrode 13 through connector 15 and wiring board 11 to actuator 7. A reference numeral 16 designates a magnetic head mounted to actuator 7 (hereinafter referred to as the movable head), and is in electrical contact with upper transformer 10 through connector 17, wiring board 11 and connector 15. A reference numeral 18 designates a concavity provided in one part of upper drum 5 to accommodate actuator 7, with space to spare to allow for adjustments in the position of movable head 16. A reference numeral 19 (FIG. 25) designates a hole for the positional adjustment of movable head 16; and 20 designates the magnetic tape which is obliquely wrapped around the outer circumferential surfaces of the lower drum 1 and the upper drum 5, so as to travel on the surfaces at a predetermined speed, coming into slidable contact with moving magnetic head 16.
FIG. 26 is a plan view of actuator 7, FIG. 27 is a cross-sectional view along line S27--S27 in FIG. 26, and FIG. 28 is a side view along line S28--S28 in FIG. 26, in which a reference numeral 21 designates a first yoke composed of magnetic material; 22 designates a first rod-shaped permanent magnet, which is fixed to first yoke 21; 23 designates a second yoke composed of magnetic material, which has on its inner surface a protuberance 23b and is mounted to first yoke 21; 24 designates a third yoke, which is made of magnetic material and mounted to second yoke 23; 25 designates a second rod-shaped permanent magnet, which is fixed to third yoke 24 in such a way that one of its magnetic poles is facing a like pole of first permanent magnet 22; 26 designates pole pieces made of magnetic material, located between second permanent magnet 25 and first permanent magnet 22 and fixed to either one or the other; and 27 designates a plate spring made of non-magnetic sheet material the periphery of which is clamped by first yoke 21 and second yoke 23, and an extension 27a of which protrudes outward through windows 21a and 23a in first yoke 21 and second yoke 23. Movable head 16 is attached to the end of the extension 27a. A reference numeral 28 designates a plate spring made of non-magnetic sheet material which is clamped between second yoke 23 and third yoke 24; 29 designates mounting pieces held by springs 27 and 28; 30 designates a bobbin fixed by means of a bonding agent 32 to mounting piece 29 at a point such that there is a gap between the inner surface of bobbin 29 and the outer surfaces of first permanent magnet 22, second permanent magnet 25 and pole piece 26; and 31 designates a coil made of coated electrical wire that is wound onto bobbin 30 and held in the annular gap G formed between the protuberance 23b of second yoke 23.
FIG. 29 is a representation of magnetic heads mounted on upper drum 5 for use with magnetic tape device in the presently used VHS format, in which the movable head 16 is used exclusively in a special playback mode in which the recorded video image data is played back in fast forward or slow motion. A reference numeral 35 designates an EP video head for the extended play mode, in which narrow tracks are formed allowing the recording of video data for an extended period on the tape, 36 designates an SP video head for the standard video recording/playback mode; 37 designates an audio head for the recording/playback of audio data; and 38 designates an erasing head for the erasure of recording tracks one by one during editing.
The following is a description of the operation of magnetic head drive assembly 7, with reference to FIG. 27.
First permanent magnet 22 generates a magnetic flux D by means of a closed magnetic circuit formed by pole piece 26, second yoke 23 and first yoke 21.
Similarly, second permanent magnet 25 generates a magnetic flux E in a direction opposite to that of magnetic flux D by means of a closed magnetic circuit formed by pole piece 26, second yoke 23 and third yoke 24.
Thus magnetic fluxes D and E cross the annular gap G in the same direction, so that the sum of the fluxes of first permanent magnet 22 and second permanent magnet 25 interlinked with coil 31.
When, under this condition, an electrical current is made to flow from contact 12 to coil 31 through electrode 13 and connectors 14 and 15, then coil 31, bobbin 30 and moving magnetic head 16 move as a unit up in the vertical (axial) direction.
This causes moving magnetic head 16 to be displaced in the transverse direction with respect to the recording track on magnetic tape 20, tracing a magnetic recording track with excellent accuracy.
FIG. 30 is a plot of the hysteresis properties of the displacement of moving magnetic head 16 versus the drive current of magnetic head drive 7, and FIG. 31 shows the recording track pattern made of magnetic tape 20 using a magnetic head drive 7 having these hysteresis properties.
As is clear from FIGS. 30 and 31, at the initial stage when magnetic head drive 7 only has been adjusted, the reference position of moving magnetic head 16 will move, due to the hysteresis properties shown in FIG. 30, resulting in an overlap .alpha. between adjacent recording tracks T.
Conventional magnetic head drive mechanisms are of the configuration described above and there is a hysteresis between head drive current and the displacement of the moving magnetic head. As a result of this, when the system is set to record after special or normal playback, the moving magnetic head fails to move to the predetermined position, creating a difference in heights between the two oppositely disposed moving magnetic heads..
As a result of this the recording tracks overlap, a good recording track cannot be obtained, and picture quality is adversely affected.
Overcoming these problems by a configuration in which a movable head mounted on an actuator is used for playback only and recording is done by a head fixed to an upper drum would require a new record-only head. More heads mounted on the upper drum would lead to increased costs, and the vibration produced when the tape is brought into contact with the magnetic head would cause a phenomenon known as head beating, with adverse effects on picture quality. The larger number of heads would also require more rotary transformer channels and thus a larger rotary transformer.